The HtrA (high temperature requirement) family represents a new class of oligomeric serine proteases in which a catalytic domain is combined with one or more C-terminal PDZ (protein-protein interaction) domains. These proteins are widely conserved and found in most organisms from bacteria to mammals.<br> In this thesis, studies were performed to obtain an understanding of the structural properties of human HtrA1 and the regulation of its proteolytic activity. Crystallization and subsequent structure solution consisted of several consecutive steps, beginning with the establishment of an efficient and reproducible protein purification procedure, followed by N-terminal sequencing, mass spectrometry, size-exclusion chromatography and dynamic light scattering in order to detect any unwanted post-translational modifications, to elucidate the molecular weight of the oligomer in solution and the homogeneity of the sample. The crystallization of the protein was performed by employing the widely used sparse-matrix screening methods. After production of diffraction quality crystals, the crystal structure of HtrA1 was solved by molecular replacement (MR).<br> To access the structural properties of HtrA1 different deletion constructs of the proteolytic active and inactive form of HtrA1 were applied for crystallization. Finally three crystal structures are presented for human HtrA1. One crystal structure shows the protease domain in the inactive conformation and two showing the protease plus PDZ-domain in the inactive and active conformation. To get the active conformation, HtrA1 was co-crystallized with a peptide inhibitor which was shown to abolish the proteolytic activity.<br> With the crystal structures it was possible to elucidate the organization of the oligomer and the structural determinants of the active site. <br> The active and inactive states both share common structural features with other members of the HtrA family. But they also show striking mechanistic differences. <br> Interestingly, binding of the peptide inhibitor to the active site of the protease domain alone leads to conformational changes of all active site loops setting up the active conformation showing defined interactions with the peptide inhibitor. This observation is different to known HtrA structures where the active site becomes ordered by specific interactions with the PDZ domain or to a stress peptide bound to the PDZ domain.<br> In the second part of the study, the development of an activity assay and the measurement of enzymatic activity of different deletion constructs was the main focus. Furthermore inhibitors, peptides and substrates were tested for the ability to bind to HtrA1 and to regulate the enzymatic activity. The role of the PDZ domains in human HtrA1 was one of the leading questions.<br> While in bacterial HtrAs the PDZ domains function as sensors of protein folding stress, binding sites for substrates, regulators of the enzymatic activity and are required for oligomerization, it could be shown that in human HtrA1 they are dispensable for proteolytic activity but are involved in allosteric regulation. Furthermore they influence the product size of cleaved substrates. <br> Most interestingly these data also suggest that human HtrA1 could be regulated by a conserved mechanism of activation by oligomerization which was first described for bacterial DegP (Krojer et al. 2008). It could not be shown for DegS or human HtrA2. It is therefore exciting to speculate that in analog to DegP HtrA1 establishes a packaging device that is able to sequester unfolded proteins for partitioning between refolding and degradation pathways. <br> Although the crystal structures of HtrA1 and biochemical analysis have enlarged our knowledge about HtrA1, there are still a great number of open questions regarding the regulation of this family member. Therefore the presented results could be the guide for future experiments that will allow a detailed view into the activation mechanism of human HtrA1.